The present invention relates to a cathode ray tube and more specifically to an improvement in the geometry a faceplate""s curved surfaces to ensure uniform brightness over an entire image displayed on a phosphor screen formed over the inner surface of the faceplate as an electron beam is projected onto the phosphor screen.
The glass envelope of a cathode ray tube generally comprises a panel portion having a curved faceplate, a neck portion with a reduced diameter, and a funnel portion shaped like a funnel to connect the panel portion and the neck portion. The cathode ray tube further includes a phosphor screen formed over the inner surface of the faceplate, an electron gun installed in the neck portion, and a deflection yoke mounted around the funnel portion. The glass envelope of the cathode ray tube has a near vacuum in its interior and has an atmospheric pressure impressed on its outer side at all times, so that the glass envelope is required to have a mechanical strength higher than a predetermined level. For this reason, various parts of the glass envelope are formed to such thicknesses as will be able to support the corresponding mechanical strengths. In a conventional known cathode ray tube, the faceplate of the glass envelope normally has a construction in which the peripheral area of the faceplate is made thicker than the central area.
FIG. 11 is a cross section showing one example of the construction of the faceplate portion in a glass envelope of a known cathode ray tube.
In FIG. 11, reference number 31 represents a faceplate, 311 denotes an inner surface of the faceplate, 312 denotes an outer surface of the faceplate, tpc a thickness of a central area of the faceplate 31, tpa denotes a thickness of a peripheral area of the faceplate 31, Rpi denotes a radius of curvature of the faceplate""s inner surface 311 with the deflection center point O of the electron beam taken as its center, and Rpo denotes a radius of curvature of the faceplate""s outer surface 312 with the deflection center point O of the electron beam taken as its center.
As shown in FIG. 11, the faceplate 31 is constructed such that the thickness tpa of the peripheral area is greater than the thickness tpc of the central area to maintain the mechanical strength as described above. As a result, the radius of curvature Rpi of the faceplate""s inner surface 311 is smaller than the radius of curvature Rpo of the faceplate""s outer surface 312, i.e., tpc less than tpa and Rpi less than Rpo.
In the above known cathode ray tube, the thickness tpc of the central area of the faceplate 31 is small and the thickness tpa of the peripheral area is large, so that when an image is displayed on the phosphor screen, light radiated outwardly from the phosphor screen through the faceplate 31 becomes attenuated more in the peripheral area of the faceplate 31 with a large thickness tpa than in the central area with a small thickness tpc. That is, if we let Tpc stand for the light transmittivity in the central area of the faceplate 31 and Tpa represents the light transmittivity in the peripheral area, then Tpc greater than Tpa and the brightness of the displayed image is lower in the peripheral area of the faceplate 31 than in the central area, giving rise to a problem that the brightness of a displayed image cannot be maintained at a sufficient level in the peripheral area. The luminance in the peripheral area is further degraded by the fact that the weight of the phosphor is smaller in the peripheral area than in the central area.
To correct the brightness of a displayed image in the peripheral area of the faceplate 31 to match the brightness in the central area when the displayed image in the peripheral area is dark compared with that in the central area, the intensity of the electron beam projected onto the peripheral area of the phosphor screen needs to be set stronger than that of the central area. Such a means for correcting the electron beam intensity, however, cannot easily be obtained.
In general cathode ray tubes, a deflection voltage applied to the deflection yoke is set as small as possible to minimize the leakage magnetic field from the deflection yoke. In recent years, however, a growing number of cathode ray tubes with an increased deflection angle have come into use. Because the deflection voltage supplied to the deflection yoke of the cathode ray tube increases with the deflection angle, it is difficult to reduce the deflection voltage applied to the deflection yoke, giving rise to the problem that the leakage magnetic field from the deflection yoke cannot be reduced.
The present invention has been accomplished to overcome the above mentioned problem and its objective is to provide a cathode ray tube which can match the brightness of a displayed image in the peripheral area of the faceplate to that of the central area with a simple means.
Another object of this invention is to provide a cathode ray tube which, even when the deflection angle is large, can reduce the deflection voltage applied to the deflection yoke and therefore reduce the leakage magnetic field from the deflection yoke.
To achieve the above objective, the cathode ray tube of this invention has the faceplate""s curved surfaces configured so that the radius of curvature of the faceplate""s inner surface is equal to or larger than that of the faceplate""s outer surface and that the black matrix hole transmittivity is defined in a predetermined range.
With the above means, because the faceplate""s curved surfaces are so configured that the radius of curvature of its inner surface is larger than that of its outer surface, the difference in thickness between the central area of the faceplate and the peripheral area becomes small and the central area is slightly thicker than the peripheral area, with the result that the brightness of the displayed image in the peripheral area of the faceplate matches that of the central area. This eliminates the need to increase the black matrix hole transmittivity excessively in the peripheral area of the screen, making it possible to provide a cathode ray tube with good color purity without significantly degrading the resolution in the peripheral area of the screen.
Further, with the above means, the geometry of the faceplate curved surfaces is such that the radius of curvature of the inner surface of the panel portion""s faceplate is greater than the corresponding radius of curvature of the known cathode ray tube and therefore the distance from the deflection center of the electron beam to the peripheral area of the phosphor screen formed over the inner surface of the faceplate is longer than the corresponding distance of the known cathode ray tube. To the that extent the radius of curvature is longer, the deflection angle of the electron beam at the deflection yoke is reduced, which in turn reduces the deflection voltage applied to the deflection yoke and therefore reduces the leakage magnetic field from the deflection yoke.